The present invention relates to a method for producing finely divided iron powder from particulate iron ore or agglomerated iron oxide pellets. In recent years the field of powder metallurgy has expanded greatly. Powdered metals are now used in dies, forging blanks and related operations, and in diverse uses such as filter packings. Powders required for these operations are produced by grinding, atomization or in long tunnel kilns or fluidized beds.
Heretofore, the shaft furnace direct reduction processes have been used only to produce pellet, lump or briquette product. Fine materials are considered an undesirable product by the operators of these furnaces, and the operating practices sepcifically are designed to avoid production of fines. Raw materials to feed such furnaces are selected for their ability to remain as lumps or pellets and to avoid decrepitation. When the entire output of a direct reduction furnace is to be utilized as feed for a steelmaking furnace, it must be in suitable form, i.e. lump, pellet, brick, or briquette, to be handled, to be readily melted, to avoid explosion and to avoid being blown out of the steelmaking furnace with the off gases. Therefore, the fines must be agglomerated, usually by briquetting, before such use as electric furnace feed. This increases the cost of producing the electric furnace feed.
I have determined that a significant and variable fraction of the output of a direct reduction shaft furnace can be a highly metallized, highly reduced, finely divided iron powder suitable for use in finely divided form, for example in powder metallurgy applications. This can be done only provided the iron powder has sufficiently small particle size to be used in such applications. Iron powder for powder metallurgy applications commands a premium price, which is much higher than the price of reduced iron feed material for steelmaking furnaces. Therefore production of even a fraction of fines useful for later application as powders can raise the average of furnace output. The finely divided powder product made in a direct reduction furnace is generally more coarse and of poorer quality than powdered metals made by more expensive techniques. However many powder metallurgy applications will allow the blending of the poorer quality fines with the more expensive powders, which can significantly reduce the cost of such powders by 10 percent or more. Machine parts which are formed from metal powders by isostatic pressure can utilize even a higher percentage of the coarse fine materials than other powder metallurgy applications. This is due to the extreme high pressures employed in the forming operation. Other uses such as filter bed are not so sensitive to size gradation. Therefore, I have developed a method of operating a direct reduction shaft furnace to produce a greater quantity of iron powder, or fines, than in normal operation of a direct reduction shaft furnace.
Volk et al, U.S. Pat. No. 3,347,659 discloses a method of making reduced iron oxide powder for powder metallurgy applications in a fluidized bed but it is first necessary to mechanically reduce the particle size to the desired product grain size, after which the powder is chemically reduced. Although it is desirable in the instant process to do some reduction in particle size prior to chemical reduction of the feed material to iron, it is only necessary to have a small amount of the feed stock of the end product particle size. Of course, only a portion of the product need be iron powder. The remainder of the product, which may be anywhere from 20 to 80 percent, will be pellet or lump product which is then screened to separate it from the iron powder. The pellet or lump product is utilized as raw lump material for uses such as electric steelmaking furnace feed or a portion of it may be returned to the direct reduction furnace if a higher percentage of the output is desired as iron powder product. Such recycling also raises the percentage of metallization, a desirable characteristic for both iron powders and lump.
The reader may also be interested in the following U.S. Patents relating to the production of metal powders: Golwynne U.S. Pat. No. 2,638,627, Syrkin et al. Pat. 3,376,129, Holtz U.S. Pat. No. 3,533,135, Lightner Pat. No. 3,692,443, Chao U.S. Pat. No. 3,744,944, Neskora et al. Pat. No. 3,877,931, and Greene et al. U.S. Pat. No. 3,910,785.